It looks like my car used to have an antenna on the rear window. There'* a base stuck on it with a threaded piece to screw an antenna onto. It looks like a mobil phone antenna. Any idea where I can get one cheap? Probably one of those black rubber ones, about a foot long maybe?

is it on the right side of the back window at the top? If so that is prolly the Onstar antenna

No, the OnStar antenna is a shark fin looking thing on the roof.

The OnStar antenna should be located where the threaded piece is on that square base right side near top of rear window.

Oh wait....the shark fin is for my XM radio. So it probably is for the On Star then....so where do I get one? Even if I don't activate it I still would like to have an antenna there.

you can prolly go to Advance/Pepboys and pick up the antenna....

Yep, that half inch cable with a thread goes to what should be a 1 1/2 squard box. the window should have 4 small dots in a square where the onstar box would go. If you dont have any tint on the window, if it is cold outside, I'd advise using a heat gun to heat the window in that location up to nuke warm to the touch, Not hot, then apply the attenna. Ive had to install cell phone[bag phones] in cars before, and found a problem with just sticking the attennas to the windows, if the climate outside gets below like 55 at the time of application. Thats just me tho. depends on where you live.